1. Field of the Invention
This invention concerns improvements in testing the pressure drop across tobacco smoke filters and smiliar rodshaped, fluid permeable articles. More particularly, this invention relates to an apparatus for testing the pressure drop across encapsulated rods of varying lengths.
2. Description of the Prior Art
In producing a cigarette with desirable physical characteristics and properties, one of the most important factors that must be taken into account is the ease with which smoke can be drawn through the cigarette.
A cigarette which is difficult to draw is ordinarily unacceptable to a smoker while one with little draw resistance will ordinarily result in increased burning and faster entry of hot smoke into the smoker's mouth with possible unpleasant irritations.
In the production of a filter cigarette, a filtering material is interposed between the smoke producing tobacco column and the smoker's mouth to remove various allegedly harmful ingredients in the smoke. The filtering material, however significantly effects the overall porosity of the entire cigarette. In fact, in the average case, the filter, while occupying about one-fifth of the overall cigarette length, accounts for one-third to three-quarters of the resistance to draw of the entire cigarette. It is therefore critical to the production of an acceptable filter cigarette to consistently produce a filter in which the ability to retain such smoke ingredients as tar and nicotine is precisely balanced against the ease of draw.
In cigarettes, the air flow is laminar. Consequently, the flow is proportional to the pressure difference across the ends of a cigarette or likewise a filter rod. This enables porosity, expressed in cubic centimeters per second at a constant pressure, to be easily correlated with pressure differences found at a standard flow rate. Accordingly, an accepted approach to measuring porosity of filter or cigarette rods entails a measurement of the prssure difference required to produce a given constant air flow rate. A simple apparatus to accomplish this measurement typically comprises a receptacle in the form of a tube to accommodate at least a portion of the rod, a vacuum source to draw air through the rod and a manometer to measure the reduced pressure which results from the pressure drop across the rod.
In addition to reflecting the ease with which air may be drawn through the rod the pressure drop measurement may alsp be used to estimate the weight of tow or other filtering material being utilized in the rod and to monitor the taste and chemical parameters of the filter.
As a result, pressure drop is often used as a primary control unit of measure in the production of tobacco smoke filters with tolerances in the range of .+-. 0.5 inches of water. Utilization of the simple pressure drop apparatus described above for such control has been found to be unsatisfactory because of the varying porosity of the cigarette paper or other wrapping material for the rod which permits varying amounts of air to be pulled radially into the filter in addition to the primary axial flow of air.
To eliminate the radial air flow the pressure drop apparatus of the prior art is provided with a rod receptacle tube sufficiently long to accommodate the entire length of the rod to be tested with a flexible sleeve mounted within the receptacle to form an air-tight seal around the length of the rod. This type of apparatus is generally referred to as an "encapsulating" pressure drop apparatus.
The encapsulating pressure drop apparatus proved satisfactory and highly successful for many years. However, in recent years there has been an explosive proliferation of cigarette brands which, in many cases, are differenciated merely by variations in the configuration or length of the filter attached to the tobacco column. Unfortunately, such prior art pressure drop measuring apparatus is designed primarily for one standard length tobacco smoke filter rod. In order to carry out pressure drop measurements on varying length filter rods stop rods must be fabricated to insert in the filter rod receptacle of the pressure drop measurement apparatus to compensate for the variation in length between the rod being tested and the conventional rod length.
While the use of rod stops does permit the utilization of a conventional encapsulating pressure drop apparatus in testing rods of varying lengths, the use of the stops presents other problems.
Initially, individual rod stops must be fabricated for each variation in filter length. Additionally, the rod stops have to be inserted in the rod receptacle tube prior to insertion of the filter rod making retrieval of the stop from the far end of the tube difficult when it is necessary to change to a stop of a different length to test filters of a different length.